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kz-550 Decently Extreme Backfiring 29 Jul 2021 09:44 #852777

  • Inspidship
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Hola folks. I threw this in the carb category mostly because I think it could be either engine or carb but ill roll with it for now.
To sum up the build so far, ive put pod filters on, cleaned up the engine, checked valve clearances, got proper 2 ohm coils (points ignition), got a 4 into 1 header with a little straight through glass pack (for now). Bike has some really really mean backfiring after letting off throttle. Foot and a half fireballs. Im assuming this isn't normal. Ill list my carb specs below just in case anyone has any recommendations regarding that. Any input is appreciated and I will be actively checking in and updating with any answers that may assist in finding the problem. Thanks a ton!

Pilot Jet - 37 (I think)
Main Jet - 97ish (#61 drill bit)
Needle - 3rd groove up
1980 KZ-550

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kz-550 Decently Extreme Backfiring 29 Jul 2021 10:27 #852780

  • ThatGPzGuy
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Just guessing but it sounds like a lean condition brought on by pods and a straight thru exhaust. What do the plugs look like? 
Jim
North GA
2016 Yamaha FJR1300ES
1982 GPz750 R1
1974 Kawasaki H1
1976 Kawasaki KZ400
1979 Yamaha XS650 cafe'
2001 KZ1000P
2001 Yamaha YZ426
1981 Honda XR200 stroked in an '89 CR125 chassis
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"If you didn't build it, it's not really yours"
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kz-550 Decently Extreme Backfiring 29 Jul 2021 13:01 #852789

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Apply some choke or enricher when decelerating. Does it help or hurt?
Turn pilot screws to maximum richness.
Raise needles to highest clip position.
Increase pilot jet size.
You may have to richen your idle circuit more than you like to ge rid of the decel pop.
Of course, foot and one half fire balls are kind of cool too.
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kz-550 Decently Extreme Backfiring 29 Jul 2021 17:35 #852810

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Messed up jetting brought on by the mods.  And points will burn with 2 ohm coils, although this is unlikely to cause fireballs out the exhaust.
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kz-550 Decently Extreme Backfiring 29 Jul 2021 18:17 #852814

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Stupid question, have you checked to make sure all your header pipes are getting warm when you start it?
78 KZ1000 A2A
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kz-550 Decently Extreme Backfiring 29 Jul 2021 18:39 #852819

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( In response to plug question) Plugs look decently black after pulling them
 
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Last edit: by Inspidship. Reason: Forgot to quote

kz-550 Decently Extreme Backfiring 29 Jul 2021 18:42 #852820

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Apply some choke or enricher when decelerating. Does it help or hurt?
Turn pilot screws to maximum richness.
Raise needles to highest clip position.
Increase pilot jet size.
You may have to richen your idle circuit more than you like to ge rid of the decel pop.
Of course, foot and one half fire balls are kind of cool too.
 

Foot and a half fireballs are super sick. The bike also seems to Peter out around 5k rpm on the main jet im guessing. Do you think richening that would have an effect?
1980 KZ-550

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kz-550 Decently Extreme Backfiring 29 Jul 2021 18:43 #852822

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Stupid question, have you checked to make sure all your header pipes are getting warm when you start it?

I didn't use that method, but if the goal is to find out if all cylinders are firing, I had a problem with one not firing, so I pulled one spark plug at a time and checked for engine noise changes. If thats not the goal, ill throw a thermometer on it and try that out
1980 KZ-550

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kz-550 Decently Extreme Backfiring 29 Jul 2021 18:44 #852823

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Messed up jetting brought on by the mods.  And points will burn with 2 ohm coils, although this is unlikely to cause fireballs out the exhaust.

I tried my best to follow jetting protocol from another forum post on here, my main jet is a little lean from what they call for though but I wasn't sure if that would affect it. Also I lied, I used to have 2 ohm coils and recently got 4 ohms because of the points
1980 KZ-550

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kz-550 Decently Extreme Backfiring 29 Jul 2021 19:48 #852826

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Foot and a half fireballs are super sick. The bike also seems to Peter out around 5k rpm on the main jet im guessing. Do you think richening that would have an effect?
 


I once had a CL response to a bike I was selling, saying the bike looked sick. I was offended, but my son assured me it was a compliment.
With your mods, I would expect several trial and error carb adjustments. Yes, see if larger jets help or hurt.
I like figuring these things out with plug chops. 10 minuets at idle. 10 minuets at steady 60mph. A few minuets at the highest speed you dare.
To reduce popping, you may need to go a little rich at idle. dark brown is OK, but not sooty black.
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kz-550 Decently Extreme Backfiring 30 Jul 2021 04:16 #852835

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Stupid question, have you checked to make sure all your header pipes are getting warm when you start it?

I didn't use that method, but if the goal is to find out if all cylinders are firing, I had a problem with one not firing, so I pulled one spark plug at a time and checked for engine noise changes. If thats not the goal, ill throw a thermometer on it and try that out



 

I'm not suggesting that you grab hold of a hot pipe but when you first start the bike up you can lightly hand tap each header pipe very quickly to see if they are all warming up.
If you have a infrared thermometer even better.
Be careful!
78 KZ1000 A2A
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Last edit: by bluej58.

kz-550 Decently Extreme Backfiring 30 Jul 2021 06:00 #852836

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Stupid question, have you checked to make sure all your header pipes are getting warm when you start it?

I didn't use that method, but if the goal is to find out if all cylinders are firing, I had a problem with one not firing, so I pulled one spark plug at a time and checked for engine noise changes. If thats not the goal, ill throw a thermometer on it and try that out




 

I'm not suggesting that you grab hold of a hot pipe but when you first start the bike up you can lightly hand tap each header pipe very quickly to see if they are all warming up.
If you have a infrared thermometer even better.
Be careful!

you can also spray a fine mist of water on each pipe and note how quickly it evaporates. Much less painful than touching the pipes.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough
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